Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP LTE systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point (AP) to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point (AP).
Often, in such communication systems, a transmitter includes several components for conditioning outgoing data for transmission via a wireless medium. Such components may include, for example, a modulator for generating complex symbols from the outgoing data, and an inverse Fast Fourier Transfer (IFFT) for converting the complex symbols from frequency domain to time domain. Additional processing may occur in the time domain, such as cyclic prefix addition, windowing, overlap, and add of windowed symbols. Then, the outgoing signal is upsampled and processed by a digital-to-analog converter (DAC) to produce an analog signal. This analog signal is then further filtered and upconverted in the analog domain to generate a radio frequency (RF) signal for transmission into the wireless medium via an antenna.
In many systems, the DAC is typically operating at a sampling frequency (rate) much higher than the baseband system bandwidth (referred to herein as the first sampling rate). As an example, in one implementation, the baseband system bandwidth can be 10 MHz and the DAC sampling frequency (rate) can be 160 MHz. The reasons for such a high sampling rate are two-fold: (1) Large sampling rate ensures that the images in frequency domain are well-separated from the baseband signal spectrum; and (2) the baseband signal spectrum undergoes minimal distortion due to the DAC low-pass “Sync” filter, by ensuring that the stop-band of the Sync filter is much higher than the baseband signal spectrum.
In many systems, a time-domain upsampler/interpolator is employed to upsample the outgoing signal to the DAC sampling rate. The time-domain upsampler/interpolator is typically a series of time-domain filters with several time-domain taps. Upsampling/interpolation involves convolving the baseband signal with these time-domain filters, which are clocked at frequencies smaller than or equal to the DAC sampling frequency. These operations are computationally intensive and consume substantial power.
As an example, the first sampling rate may be 10 MHz, and the DAC sampling rate may be 160 MHz. The time-domain upsampling/interpolator needs to upsample the time domain from 10 MHz to 160 MHz—a factor of 16. This can be computationally intensive. Accordingly, it would be desirable to completely eliminate the time-domain upsampler/interpolator or significantly reduce its complexity, while at the same time, achieve the desired sampling rate for the transmitted signal.